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The case for the use of laboratory bioassay in the regulatory control procedure was discussed using vinyl-chloride (75014) as an example. The validity of laboratory bioassay as a system for predicting carcinogenic and mutagenic hazard in humans was considered. Experiments with animals in the early 1970s revealed that vinyl-chloride could induce cancers in a site specific manner, particularly angiosarcoma of the liver. When three cases of liver angiosarcoma were reported in 1974 among workers at a vinyl-chloride polymerization facility, NIOSH undertook an epidemiological investigation. The NIOSH study used a cohort restricted to individuals with at least 5 years exposure to vinyl-chloride and at least 10 years elapsed time since the initial exposure. A significantly greater number of total neoplasms was observed in the cohort compared to the expected number. At each specific site the risk of neoplasm was increased in workers with 15 years since onset of exposure compared to workers with only 10 years since onset of exposure. Histopathological examination of tissue specimens revealed the presence of angiosarcomas in 11 out of the 14 cohort members who had died of liver cancer. Genetic effects have been shown to occur in bacteria, yeast, insects, plants and mammals as a result of exposure to vinyl-chloride. A NIOSH study on the incidence of fetal loss in wives of exposed workers showed a significant association between fetal loss and vinyl-chloride exposure. The predictive value of carcinogenesis bioassay was discussed in relation to other chemicals for which evidence for carcinogenicity first came from animal studies. The authors conclude that the case for use of laboratory bioassay in the regulatory control process has been demonstrated.